I think the Senn MX300/400/500 series change their sound with use. After about 40 hours, I noticed my MX400's sounded a little warmer, as if the driver diaphragms loosened up. Just don't blast music too loud during this period. I've noticed that too much volume too quick tend to make them sound crackly, or staticy on some music (particularly with lots of guitars).

yeah, blasting them before you burn them in results in a crackly sound.... it's like the drivers haven't been loosened up enough to take the volume yet... at least that's what's happened with the 3 pairs of Senn MX series buds that i've had...

I guess I'll be the first to add that you should run over to a Raido Shack and pick up some replacement foamies. They're around $2.00 and make a HUGE! difference in the sound, most noteably in the bass department.

Just a note on the RS foamies, they seem to be the same as the Sony foam covers, and are a little thicker than the Senn covers. Depending on your ear size they generally make a better seal and give more volume and bass due to better coupling.

I have (had) the MX-300, 400, and 500 Senns. The 300s are more bassy but the 400s are better overall ($10 shipped). I think the 500s have a little better resolution and the inline volume control is OK too.

My overall preference is the Sony MDR-E848LP Fontopia. They have a better shape and thus better coupling, as well as more definition, perhaps due to the PET (titanium) driver diaphragm.